U.S. Pat. No. 3,827,375 discloses railway flat cars having container pedestals including locking members which hold piggyback containers in place on railway flat cars. The locking members automatically pivot about a horizontal axis, overcoming the bias of a stiff spring, as containers are loaded and unloaded from the pedestals. A recent Association of American Railroads (AAR) specification covering latches of this type in part requires that if the hold-down device is of the self-entry or self-release type, the design should be such as to permit release of the container corner fitting when the container is pulled up through the device with a force of not less than sixteen hundred (1600) pounds nor more than twenty-two hundred (2200) pounds per corner. Self-entry or self-releasing type hold-down devices should permit engagement of the container corner fittings with entry force of not more than eight hundred (800) pounds per corner.
In testing the latches constructed in accordance with the foregoing patent with regard to compliance with the foregoing specification, it was found that the latches did not consistently meet the twenty-two hundred (2200) pound maximum exit force required to remove a container from the latch. In many cases a force considerably in excess of the twenty-two hundred (2200) pound maximum per corner was required to remove the containers from the latch. However, generally the latches did meet the minimum of sixteen hundred (1600) pounds exit force and the eight hundred (800) pound maximum per corner entry force.
One solution we attempted was to apply grease or a lubricant to the latch protuberance which would lower the coefficient of friction and thus the force required to remove a container from the latch. However, this solution would require periodic application to each of the latches on a railway flat car, usually sixteen (16) in number, and thus would be a maintenance expense and problem to be certain the lubricant was applied. Furthermore, the lubricant lowered the minimum exit force to below sixteen hundred (1600) pounds and thus out of specification.
Another solution we attempted was to apply a zinc coating to the latch protuberance to reduce the coefficient of friction and thus the maximum force. The zinc does not form a satisfactory coating to withstand the entry and exit contact with the container.